While the subject invention relates to an improved sensible heat storage unit, it has particular utility in solar energy conversion systems, and especially those which use Brayton cycle engines. It is therefore useful to describe the solar energy conversion system for which the subject unit was designed. Although designed for a special type of solar energy conversion system, the subject storage unit may be utilized in any application where constant discharge temperatures are required.
With respect to solar energy conversion systems, while "latent" heat switchable storage units have been utilized in steam cycle solar energy conversion systems as illustrated in U.S. Pat. No. 2,933,855 issued to E. K. Benedek et al., Apr. 26, 1960, the benefits of a "sensible" heat storage system have heretofore not been utilized with a Brayton cycle system involving a gas cycle turbine.
One of the major problems with solar energy conversion systems utilizing steam is, in general, the extremely corrosive nature of superheated steam and the upper temperature limit associated with the tubing or plumbing used. In general, it is possible to heat up solar receivers to temperatures in excess of 2500.degree. F. in situations utilizing a central receiver positioned at the focus of a mirror field which redirects sunlight onto the solar energy receiver. Thus, the capability of central receiver type installations far exceeds the restraints on superheated steam systems which, in general, must operate below 100.degree. F.
It will be appreciated that even the highest quality steels have limited strength at temperatures over 1650.degree. F. and, therefore, new types of solar receivers and storage equipment are necessary if solar energy is to be efficiently converted into electrical energy. It will be appreciated that the higher the temperature of the working fluid or gas, the more efficient will be the conversion process.
In the present illustration an air or Brayton cycle system is used instead of a steam cycle. Brayton cycle engines have the advantages of proven outstanding realibility and efficiencies 10-20% higher than the steam cycle engines. As will be seen, they integrate well with low cost sensible heat storage units, and become optimum at very low pressure ratios, which allows even higher reliabilities and high component efficiencies.
The solar energy conversion system described can withstand the high temperatures associated with central receiver type installations in which the receiver may be of the type that utilizes a ceramic honeycomb heat exchanger and in which sensible heat storage units of refractory materials are used so as to withstand the high temperatures.
In one embodiment, an efficient "split cycle" solar energy system includes a solar energy loop isolated from a Brayton cycle engine (turbine) coupled to an electric generator. In the solar energy receiver loop a switchable sensible heat storage unit is charged by the solar receiver. The charged storage unit is then switched into the Brayton cycle turbine loop where it serves as the prime energy source of the engine. Finally, after it has been discharged, the unit may be utilized as a high temperature, high efficiency recuperator to recover waste heat from Brayton cycle turbine exhaust. The switchable sensible heat storage unit system is alternatively referred to as an "energy shift register" system.
The use of the sensible heat storage unit as a recuperator permits the Brayton cycle engine to be run at extremely low pressure ratios and a thermal/electric conversion efficiency in excess of 60%. This can result in a solar/electric conversion efficiency in excess of 40%, as contrasted with steam cycle solar energy conversion efficients of less than 20%.
In the conventional Brayton cycle, large pressure losses occur in the heat addition cycle because heat is being added to a high velocity fluid stream. Also, penalizing temperature and pressure losses occur in the large recuperator needed to make low pressure ratio engines operate at high thermal efficiencies. In some types of ceramic wheel heat exchangers, there is significant leakage to further penalize performance.
The "energy shift register" system utilizing sensible heat storage improves the efficiency of Brayton engines by minimizing these losses. Heat addition occurs efficiently and slowly without significant pressure loss in a large insulated tank filled in one embodiment with alternated materials of different thermal conductivity which produce low conductivity in the flow direction. In one embodiment the storage unit is formed by spaced ceramic matrices or perforated ceramic elements. As the air passes through the matrices at velocities of 1 m/sec or less, a sharp thermocline (called herein a "step function" thermal gradient) develops; i.e., in a narrow region of the tank a major temperature gradient develops, and travels at approximately 1/1000 of the air velocity. As will be seen, this permits discharge of the tank at a uniform temperature. When the step function thermal gradient reaches either the top or bottom of the tank, the tank is considered full and must be switched out of one position of the Brayton cycle into another. Hence, the name "energy shift register."
As can be seen, the sensible heat storage is utilized to isolate the receiver loop from the engine or electric power generating loop. Thus, the sensible heat storage unit provides a large buffer for the turbine and allows a high degree of flexibility in plant operation by allowing different rates of thermal energy collection and consumption.
In short, the isolation between the receiving loop and the engine loop buffers the engine against changes in solar flux due to the passing of clouds over the sun, etc., or from any receiver-related condition. Thus, the engine loop can be made and designed to run at maximum efficiency regardless of the operating conditions in the receiver loop.
Moreover, because of the isolation between the receiving loop and the engine loop in the subject invention, the solar receiver loop may operate at a different pressure than the engine loop, since the storage unit to be described can be discharged at any desired pressure. Separating the receiver from the pressurized engine loop permits the use of an "open-ended" ambient pressure solar receiver in which a "window" need not be used. The "open-ended" receiver typically operates at ambient pressure to reduce sealing requirements and for safety and low cost. This receiver also uses air which is a non-polluting working fluid. Moreover, when working at atmospheric pressure, the heat exchanger in the receiver may be assembled loosely to its housing to allow room for thermally induced motions.
While a split cycle solar energy conversion system with sensible heat storage has been described in which a Brayton cycle engine is utilized, it will be seen that the subject system involves improvements in the Brayton cycle system itself. The improvements to the Brayton cycle system include the use of a sensible heat storage unit both for recuperation and as a prime energy source.
As a prime energy source, operating the storage unit at low pressure makes it possible to run the Brayton cycle engine at highly efficient low pressure ratios. Moreover, energy for the Brayton cycle engine may be provided not only from the sun, but also from extremely "dirty" fuels. This is because deposits from the fuels are not picked up by the low velocity gaseous working fluid and do not reach the Brayton turbine blades to corrode them.
When the sensible heat storage unit is used as a recuperator, because of its extremely high effectiveness, the entire efficiency of the Brayton cycle system is significantly increased.